It's night and you want to figure out the time by reading the positions of the stars. Before the invention of clocks, people told the time by the movement of the stars across the night sky. You can do the same using your star clock.

What you do:
Carefully cut around each star clock circle and poke a hole through the middle of each one. Place the small circle on top of the large circle. Push a paper fastener through the holes in both circles and spread the fastener open on the back of the clock. Go outside, look up at the sky, and using your star clock find the Big Dipper and the North (or Pole) Star. Face the North Star. Put your thumb over the current month. Slide the outer circle around so that your thumb is at the top. Turn the smaller disc carefully until its stars line up with those in the sky. You can now read the time in the window. (If you are on Daylight Savings Time, add one hour.) Compare the time with your wristwatch to see how close you get. It's better to do this activity when the moon is not full. A full moon is so bright that it becomes difficult to see the stars.

What's going on?
The North Star never appears to move because the Earth's axis, the imaginary line drawn from pole to pole through the center of the Earth, points almost directly to the North Star. The stars that appear to revolve around the North Star are known as circumpolar stars. In mid-northern latitudes, these stars appear to circle around the North Star without rising or setting. The star clock estimates the time based on where the stars appear relative to the North Star.

Activity adapted from Lawrence Hall of Science. Earth, Moon, and Stars. Regents of the University of California, 1986.

For more information, see Rough Science episode 7: "Mediterranean Mystery"

Make a Telescope

When you first look up to find the Big Dipper to orient your star clock, you may have trouble seeing it. Why don't you make sure you'll find it by making your own telescope? Even if you find the Big Dipper and the North Star with ease, your telescope will help you to see the moon and thousands of other stars in much greater detail.

a cardboard tube at least as long as the sum of the two focal lengths of the lenses

pen

tape

What you do:
Fix one lens to each end of the tube with tape. Take care not to obscure the view through the tube. Mark the end of the tube with the shorter focal length lens. This will help you figure out which way round your telescope is. Look through this end.

What's going on?
Telescopes use lenses to bend the incoming light. The first lens (objective lens) gathers light and bends it into focus and provides a small, upside down image of the object you're looking at. The second lens (the eyepiece) then magnifies the object so that you can see it better. When the two lenses are combined, you have a telescope that magnifies the image.